[glibmm] tools: Add docs to the default constructor of _CLASS_OPAQUE_COPYABLE.

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 045278701c34fafb0de16f9165316d3a87758313
Author: Kjell Ahlstedt <kjell ahlstedt bredband net>
Date:   Tue Jul 23 16:03:03 2013 +0200

    tools: Add docs to the default constructor of _CLASS_OPAQUE_COPYABLE.
    
    * tools/m4/class_opaque_copyable.m4: Add documentation to the generated
    default constructor, mentioning that the constructed object is invalid.
    Bug #704639.

 tools/m4/class_opaque_copyable.m4 |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/tools/m4/class_opaque_copyable.m4 b/tools/m4/class_opaque_copyable.m4
index 81be7b5..9e53649 100644
--- a/tools/m4/class_opaque_copyable.m4
+++ b/tools/m4/class_opaque_copyable.m4
@@ -167,6 +167,10 @@ public:
 
 ifdef(`__BOOL_CUSTOM_DEFAULT_CTOR__',`dnl
 ',`dnl else
+  /** Constructs an invalid object.
+   * E.g. for output arguments to methods. There is not much you can do with
+   * the object before it has been assigned a valid value.
+   */
   __CPPNAME__`'();
 ')dnl